This study proposes to evaluate the utility of measuring the effects of low level, long term occupational exposure of hospital personnel to antineoplastic agents through conventional cytogenetics (chromosomal breakage) and to attempt to correlate those measurements with micronuclei frequency. The study population includes nursing personnel evaluated for actual occupational exposure to antineoplastic agents through admixing and administering chemotherapy drugs or handling body fluids of patients undergoing cancer treatment with cytostatic drugs. Based on documentation of exposure, nursing personnel will be placed in one of three exposure categories: no known exposure, low exposure or high exposure. Patients diagnosed, but not yet treated for breast cancer will serve as the second study population group. Patients will be both no exposure - negative controls (pre-treatment) and very high exposure - positive controls (post treatment). All study subjects will be interviewed to identify possible confounding factors of exposure - age, working and occupational or environmental exposure to genotoxic agents. Blood samples will be collected from all subjects (single sample from nursing personnel, pre-treatment and post treatment sample from patients). Peripheral blood lymphocytes will be analyzed for determination of frequencies of chromosomal breakage and micronuclei. Frequencies will be compared between exposure groups to attempt to identify the absence or presence of a dose response relationship between exposure to antineoplastic agents and chromosomal damage. Correlation will also be assessed between the frequency of of chromosomal breakage and the frequency of micronuclei. Significant positive correlation coefficients would indicate the appropriateness of estimating chromosomal damage through the more efficient and cost effective technique of micronucleus methods.